Oct 3, 2008

Science - Of two papers and an elusive particle (G.Read)

A. Rangarajan

Interview with Professors Francois Englert and Robert Brout .

When you start talking about the massive scalar boson and the phenomenon of spontaneous symmetry breaking, Professors Francois Englert and Robert Brout of the Free University in Brussels, both in their seventies, acquire a child-like glow in their faces. It is as if the wonder they stumbled upon 45 years ago has never ceased to fascinate them. Professor Brout recalls with awe the profundity of the moment when realisation dawned on them that they were on new frontiers of theoretical physics. The results of their collaborative research made them look into the origin of massive particles associated with fundamental fields mediating short-range interactions. In contrast to the massless photon associated with the electromagnetic field mediating long-range interactions, they were on new ground, defying established notions. This was path-breaking. While at the Free University in Brussels they were wrestling with the mathematical equations that could give them insight into short-range interactions encountered in subatomic structures in terms of fundamental forces, unknown to them another Professor at the Tait Institute of Mathematical Physics attached to the University of Edinburgh was working on the same problem. That was Peter Higgs. While the Brussels-based pair used a scalar field pervading the universe and in so doing constructed a mechanism capable of generating short-range interactions, remarkably Professor Higgs came to the same conclusion and even concurred on spontaneous symmetry breaking as the underlying explanation. His paper followed in a month. Then, somehow, the scalar field, the associated massive particle and the mechanism all came to be named after Professor Higgs, and not all three of them. The other two say Professor Higgs himself has always been fair in acknowledging that their contributions were equal and that the publication of their work preceded his. They attribute it to circumstances, where terms employed by the early users gained currency.The work of Brout, Englert and Higgs is now in focus because there is an opportunity to experimentally verify the existence of the BEH boson. It is to be one of the main experiments at CERN’s Large Hadron Collider in Europe. The detection of the BEH boson, dubbed rather dramatically as ‘God Particle,’ will be a breakthrough in our understanding of physical reality. It will also ground the Standard Model on a firmer foundation. (The name boson itself stands in honour of Indian physicist Bose who along with Einstein developed the Bose-Einstein statistics in quantum mechanics.) The Standard Model unifies three of the four forces known in nature — the electromagnetic, the weak nuclear and strong nuclear — leaving out only gravitation. Further, the profundity of the idea of the BEH field assumes significance in the light of the fact that it talks about a kind of mysterious presence permeating space, in some key sense even remaining indistinguishable from it. Building on this concept, more ambitious theories could try to account for how the universe burst forth from a kind of nothingness.All three physicists shared the 2004 Wolf Prize for the experimental verification of the BEH mechanism. If the scalar boson is detected at the LHC, they could come together again to share the Nobel Prize. Professor Englert and Professor Brout, could you tell us about the significance of your work and that of Professor Higgs’ in terms of the important questions in theoretical physics and within the context of the progress we have made?Englert: Physics as we know it is an attempt to interpret diverse phenomena as particular manifestations of general testable laws. Galileo, Newton, Maxwell, Einstein, and scores of others have taken us along this journey, raising hopes gradually of a possible unified theory. In this quest we encounter two phenomena, long-range interactions (gravity and electromagnetism) and short-range interactions encountered only within the nucleus of the atom. In the 1960s, theoretical interpretation of short-range fundamental interactions posed certain insurmountable problems. The breakthrough came from the notion of spontaneous symmetry breaking wherein short-range interactions are generated from long-range ones. Our work and then independently that of Higgs, thus allowed the theoretical analysis of short-range forces by unifying it in a common framework for both kinds of interactions. This was explained in terms of the BEH mechanism where both kinds of forces are transmitted by the so-called gauge field. In the case of short-range forces the gauge fields acquire mass from spontaneous symmetry breaking, making use of the scalar boson. On the contrary, the photon which is massless is associated with the electromagnetic interaction that occurs over large distances. This discovery thus permitted the application of laws known at the macro level to the sub-nuclear world.[ Interviewer’s note: Actually there are two known short-range fundamental interactions: the weak and the strong forces. The BEH mechanism is applicable as such to the former and an alternative mechanism is applicable to the latter. Nevertheless, the alternative mechanism can also be put in the BEH framework using the so-called “dual” description.]Brout: We were inspired by the work of Professor Nambu, who first suggested that spontaneous symmetry breaking already known in solid state physics could be applied to particle physics and fundamental interactions. That was deep insight and we started from there. The later work of Weinberg and Salam that led to the discovery of W and Z bosons and the formulation of the Electroweak theory (for which they won the Nobel Prize with Glashow) is confirmation that we are looking in the right direction. W and Z bosons are heavy particles associated with short-range weak nuclear forces. Their work was also a landmark moment in the quest for unification as it brought electromagnetism and weak nuclear forces on a common frameworkWe know both of you and Professor Higgs came to the same conclusions. Was there any difference in the approach?Englert: We based our analysis and mathematics on quantum mechanical methods, and in Professor Higgs’ work the model he discusses is in the classical limit of Lagrangian field theory. There are also certain distinctions in terms of abelian and non-abelian frame-works – this might require a longer explanation, but essentially the conclusions reached along the two paths are identical. In a way both papers complement each other. That brings us to the question of how physicists communicate their work and findings to ordinary people. Is this not getting difficult progressively?Brout: I think one needs to qualify the progressive element here. If you look at the assumptions Newton had to make to explain the principle of conservation of momentum in terms of the assumptions implicit on space and time, that too was difficult to follow. Even in the pre-relativity era. Galileo’s inertial principle was also hard to follow in the beginning. We need to resort increasingly to mathematics because of the precision and economy it offers as a language when we deal with scientific questions. It may be hard, but nevertheless it is possible for ordinary people to understand the issues in theoretical physics.Englert: The physicist and the interested larger audience need to take a lot of effort and then these ideas can be communicated, at least at the conceptual level.Can the theory of spontaneous symmetry breaking in the context of the BEH field be seen as a theory of mass as well?Englert: The work of all three of us essentially involves understanding a mechanism that could give rise to short-range forces from long-range forces through spontaneous symmetry breaking. More precisely, it comes through the fact that the fields which are responsible for the interactions give rise to short-range forces under spontaneous symmetry breaking by acquiring mass. Now not only do these vector gauge fields acquire mass but certain elementary particles like fermions [do so] as well. That is why it sometimes is seen as a theory for mass in very general terms.Brout: We are only talking about the mass of a certain class of particles, not all. That other elementary particles also acquire mass is part and parcel of the symmetry breaking scenario and is most easily and naturally implemented by appeal to the scalar boson.Now what happens if this boson is not found at the LHC? Professor Hawking’s wager on this is in the news. What will be the consequences for physics?Englert: Maybe Prof. Hawking has an intuition, we do not know. We have no predictions to offer. Let us wait and see. If the scalar boson is found, it means that we have the most simple and elegant expression of spontaneous symmetry breaking. If it is not found, we have to continue our search for a more complex expression. Also, if the scalar boson is found to be an elementary particle (a non-composite) then it may have wider consequences for our understanding of super-symmetry. This goes beyond the issues we have been discussing here.